The present invention relates to a process for facilitating non-cutting cold working of a ferrous material by an application of a phosphate coating by dipping in an aqueous acid phosphating solution, which contains zinc ions, Mg ions and phosphate ions as well as oxidizing agents and is virtually free of Fe(II) ions.
Phosphate coatings are usually applied to metal surfaces to improve their resistance to corrosion and to improve the adhesion of the subsequently applied paint. Phosphate coatings serve also to facilitate the non-cutting cold-working operations and in that case act themselves as a "lubricant" which avoids a seizing or welding of the workpiece material and the tool or they act to bind a subsequently applied lubricant so firmly that it virtually will not be removed by the shaping operation. Particularly the last-mentioned property is of special significance, because only the combination of the phosphate coating and the lubricant permits a repeated strong cold working, possibly without a new intermediate treatment with a lubricant.
Numerous processes for facilitating cold-working operations by an application of phosphate coatings are known. They may belong either to the category of the "layer-forming" processes or to the category of the "non-layer-forming" processes, although the latter are much less significant.
In "layer-forming" processes, phosphate coatings are formed by phosphating solutions which, in addition to the phosphate ions, contain also a major part of the cations used to form the coating. On the contrary, in the so-called "non-layer-forming" processes the cations of the phosphate coating usually come from the metal which is being treated and the phosphating solution usually supplies only the phosphate ions.
For instance, EP-A-45110 describes a process of forming phosphate coatings on iron or steel surfaces by a dipping or flooding process, in which phosphating solutions are used which contain at least 0.3%, by weight Zn, at least 0.3%. by weight PO.sub.4, and at least 0.75% by weight NO.sub.3 or an equivalent accelerator which does not oxidize iron(II). The Zn:PO.sub.4 weight ratio should exceed 0.8 and an iron(II) content of 0.05 to 1% by weight should be adjusted. The solutions described in this reference may contain calcium, which may be replaced entirely or in part by magnesium, and can be used to form phosphate coatings, i.a., in preparation for cold-working operations.
The process outlined hereinbefore is carried out "on the iron side". Any advantage whatever which might be afforded by the particularly emphasized calcium content of the phosphating solution has not been mentioned.
EP-A-403 241 describes a process for forming zinc phosphate coatings on metal surfaces by means of aqueous zinc phosphate solutions, which contain 2 to 20 g/l zinc, 5 to 40 g/l phosphate, and silicotungstic acid and/or silicotungstate in a concentration of 0.005 to 20 g/l (calculated as W). The phosphating solutions may contain nitrite, nitrobenzene sulfate, hydrogen peroxide, nitrate, and chlorate as an accelerator. The phosphating solution may additionally contain nickel, cobalt, calcium, and manganese as well as 0.5 to 10 g/l magnesium. The process can be used, inter alia, to prepare metals for cold-working operations.
A disadvantage of that process resides in that the tungsten contained in the phosphating solution will necessarily enter subsequently used rinsing baths so that problems arise in connection with the treatment of waste water.
Finally, the phosphating process disclosed in EP-A-414 301 uses phosphating solutions which contain 0.4 to 30 g/l zinc, 4 to 30 g/l P.sub.2 O.sub.5, 5 to 50 g/l NO.sub.3, up to 10 g/l Fe(II), and up to 0.3 g/l Fe(III). The solutions may also contain up to 10 g/l magnesium, inter alia, and are replenished in a specific manner and operated with a specific oxidizing additive so that the real object of that process, to permit a processing substantially without a formation of waste water, can be achieved. The magnesium content of the phosphating solutions or their calcium content, which is allegedly equivalent, affords the advantage that the coatings, which contain mixed phosphates have a higher resistance to alkali and, for this reason, are particularly suitable as primers for paints.
The phosphating processes discussed hereinbefore and most other phosphating processes have in common that they use nitrate, nitrite and/or organic nitro compounds, such as nitrobenzene sulfonate, as an accelerator. But such compounds give rise to problems in the treatment of rinse washings and waste water, because they can be removed and decomposed only with difficulty.